Algae have been grown in mass culture since the 1950s as a source of fertilizer (Burlow, 1953) and since then as a food source for both humans (Gershwin & Belay, 2007) and animals (Lundquist et al., 2010), and for use in water treatment (Oswald & Gotaas, 1957). However, recent interest in third generation biofuels has once again increased interest in the mass culture of algae for the creation of biomass for oil and lipid extraction (Oswald & Goluke, 1960). Furthermore, the potential use of algae for the long-term sequestration of carbon (Campbell et al., 2009) has highlighted the need for improved methods of algal mass culture (GB 2464763 published May 5, 2010, incorporated herein by reference).
Initially, commercial algae were cultivated using existing water treatment infrastructure, which consisted of natural ponds (e.g. Dunaliella Salina, Hutt Lagoon, Australia) and circular open air water treatment facilities (Harder and Von Witsch, 1942). However the rate of algal growth and biomass yield was found to be limited by factors including sub-optimal mixing dynamics and inconsistent gas exchange. Oblong, high-rate ponds known as raceway ponds were then developed, in which algae could be grown at a higher density (Benemann & Oswald, 1996). Raceway ponds are open ponds in which algae and water circulate around a track. Paddlewheels are used to maintain the flow of water and to keep the algae circulating to the surface and mixing the entire water mass to exchange gases with the atmosphere at a high rate. However, sub-optimal growth rates are still a problem and have prevented the mass culture of algae in raceway ponds from being commercially viable, especially on the scale required for biomass production for producing fuels (Sheehan et al., 1998), and for carbon sequestration (GB 2464763) and for the bulk production of commodity biomass production.
A raceway pond's prime advantage is its simplicity, with low production and low operating costs. However, since raceway ponds are usually completely open to the ambient environment, adverse weather (Vonshak et al., 2001) can stunt algal growth and contamination from outside organisms often results in undesirable species monopolising the cultivated algae (Ben Amotz, 2003). Contamination can be prevented by regularly cleaning the ponds, but this interrupts growing and harvesting the algae. Chemical signalling by algae, for example extracellular polysaccharide release by diatoms (Hoagland et al., 1993) as well as monosaccharide and amino acid release (Granum et al., 2002) may provide a fertile ground for bacteria and other contaminants to grow or allelopathic growth inhibiting compounds (Prince et al., 2008) can slow or stop the production of the desired algae. Furthermore, it can be very difficult to control other environmental factors such as the temperature of the water, light colour and the light intensity, resulting in photoinhibition of algal growth (Goldman, 1979).
Alternative methods have been developed in which the environmental conditions can be more closely controlled. A photobioreactor (PBR) is a closed system in which algae are grown in a controlled environment that is isolated from the ambient atmosphere, and forces the algae through thin layers to maximise their solar exposure. The exchange of gases, the addition of nutrients, algae dilution and the removal of waste products can be carefully controlled. Contamination of the algae is also minimised.
The primary objective of the highly controlled growth environment of a PBR is to achieve a very high density of algae in order to maximise productivity (Miyamoto & Benemann, 1988). However, disadvantages arise due to the high algal density. The need to constantly control parameters such as temperature, solar irradiation, exchange of gases, the addition of nutrients and the removal of waste products requires sophisticated and expensive technology. Furthermore, the unnaturally high algal densities can lead to the formation of biofilms and surface adhesion of the normally waterborne algae, which are exhibiting stress responses whereby the algae form aggregates to reduce the environmental stress of the artificial growth environment (Sukenik & Shelaf, 1984). The formation of biofilm can make a PBR difficult to clean and can clog equipment (Ben Amotz, 2008), and the high algal density can actually change the physiology of the algae resulting in the counterproductive reduction of product yield and product characteristics.
Controlling the growth environment of the algae comes at significant cost. Closed Photobioreactor (PBR) systems operate in transparent tubing, plastic bags or other walled containers. The physical drag of the water alone creates enough friction to negate the benefits of the additional yield due to the cost associated of this friction. The energetic cost of many of these systems exceed the light energy captured by the photosynthetic organisms, thereby being fundamentally unsustainable (Stephenson, et al. 2010). In addition, in high insolation environments, these closed reactor environments be they made of panels, tubes or bags experience significant heating causing new growth challenges.
In comparison, algae cultivated in open raceway ponds are usually managed to operate at relatively lower cellular densities than in PBRs, although both systems still maintain an artificially high density of algae relative to those commonly found in nature. The cell density of algae in open raceway ponds can often reach several million cells/ml in order to maintain a high standing stock of algae to supply a continuous stream that can be harvested more effectively. The density of algae cultivated in raceways ponds is often high enough that the algae respond in deleterious unforeseen ways, for example through quorum sensing where the algae aggregate to form floc that suddenly settles out in the ponds (Taraldsvik & Myklestad, 2000).
Paradoxically, natural, very large scale blooms of algae are well known and regular phenomena throughout highly diverse aquatic environments. An algal bloom occurs when a species of algae dramatically increases in number such that large areas of the ocean are numerically dominated by the algae. In comparison to the artificially controlled raceway pond and PBR growth systems described above, these natural algal blooms form in highly dynamic and complex environments, with a background of many other competing species and predators. Yet when the conditions are favourable, the algae grow exponentially, often doubling daily for two to three week periods (Platt & Subba Rao, 1970).
During these bloom events, the cellular physiology optimizes growth and maximizes the photosynthetic quantum yield, the efficiency of converting light into biomass to near theoretical maximum level of 8% for an extended period during the day (Maranon 2005). This temporary effectiveness of converting solar energy into chemical energy is a key component enabling the algae to grow exponentially. During the time of exponential growth these algae grow at a faster rate than at any other time in their lifecycle (Barber et al., 1996). They achieve the high rate of photosynthesis during a given time of day where the algae clearly distinguished stages of photosynthesis, growth and cellular division synchronized throughout the diurnal light-cycle. Algae exhibit near maximum rate of photosynthetic quantum yield, throughout the morning and through the early afternoon (Babin et al, 1995), whereas in the evening and at night time the cellular machinery is geared towards cellular growth and duplication. Consequently, during the exponential growth phase algal cellular division exhibits diurnal rhythmic behaviour (Bruyant et al., 2005).
For some species, such as diatoms, the algal cells experience wind driven mixing of the oceanic water column, while for other species such as dinoflagellates, these blooming events are combined with daily migration to deeper nutrient-rich water at night time (Eppley & Harrison 1975), followed by migration to shallower depths where more light is available for more effective photosynthesis (Kiefer & Lasker 1975). This has important consequences, as cells may be driven to have asynchronous vertical migration that maintains the growth of the bloom for a longer period of time. This cellular migration also positions the cells in the light field and nutrient regime optimally suited to a low nitrogen to carbon ratio physiological state that maximises cell mass while maintaining exponential growth (Ralston et al., 2007). Eventually, these favourable natural conditions change and the blooms collapse due to nutrient limitation, a change in the environmental conditions or predators.
The response to external stimuli such as salinity, temperature, nutrient, light or population dynamics change the algae's physiology and behaviour. The responses are directly dependent on the intensity, frequency, and combination of these environmental factors (Feng et al., 2008). Bloom-forming algae have the ability to integrate the signals they receive from these environmental signals and acclimate to the bloom-supporting conditions (Sanudo-Whilhemy et al., 2004). For example, the nutrient physiology of bloom forming diatoms (Pseudo-nitzschia sp.), raphidophytes (Heterosigma akashiwo) and dinoflagellates (Alexandrium sp., Ceratium sp), favours very rapid uptake of nutrient (high-affinity for nitrogen species) unlike many similar non-bloom forming species in the same genus (Kudela et al., 2010). In between blooms, these bloom-forming species can switch to an entirely different physiological mode where these species have much slower nutrient uptake kinetics, similar to other non-bloom forming species in the same genus.
To date, all artificial attempts to create more efficient algal growth have not attained anywhere near the efficiency of light, nutrient and resource exploitation, or photosynthetic quantum yield recorded during these natural algal blooms. The reason, we believe, is that these systems have been built to optimise environmental parameters that are not related to these bloom forming conditions. Instead, too much consideration has been given to building growth systems that address other constraints. For example, systems have been designed to grow algae with high lipid content, on expensive land (e.g., California) or high-insolation and low-water environments (e.g., Arizona), in wastewater, produced-water (saline water from mining activities) or freshwater (Sheehan et al., 1998). High cell densities to reduce the cost of harvesting, very shallow tanks to prevent self-shading and promote gas exchange, continuous growth conditions, continuous harvesting to effectively utilize the harvesting equipment, and titrated nutrient addition have all been engineered to maintain control of these growth systems. In addition, these growth systems are populated with a narrow set of hardy organisms (e.g., Dunaliella salina, Haematococcus sp.) that can tolerate hypersaline and high temperature conditions that occur in these closed systems.
Thus, there remains a pressing need to develop a method and apparatus for the efficient mass cultivation of algae that is commercially viable, yet simple and inexpensive.